1. Field of Invention
The present invention relates generally to a semiconductor wafer processing apparatus. More specifically, the invention relates to an apparatus for providing a controllable, uniform temperature distribution across the diameter of a semiconductor wafer.
2. Background of Prior Art
In semiconductor wafer processing, the surface temperature of the wafer is a critical process parameter. Changes in, and gradients across the wafer surface during wafer processing are detrimental to material deposition, etch rate, feature taper angles, step coverage and the like. It is essential to have control over local temperature uniformity during wafer processing to enhance processing and minimize undesirable characteristics.
A number of devices have been used in the art to control wafer temperature during processing. One method feeds a chilled fluid through a wafer support pedestal during wafer processing. The fluid removes heat from the wafer support pedestal thus cooling the wafer. This method of cooling the wafer has two inherent problems. First, the response times required to bring a wafer to a desired temperature is relatively long. As such, rapid dynamic control of the water temperature to compensate for rapid wafer temperature fluctuations is not possible. Consequently, the wafer is not maintained as a constant temperature. Typically, the response times are limited to approximately 2 minutes per degree.
A second disadvantage of this method is the inability to control the temperature uniformity across the surface of the wafer. Heat transfer from the wafer to the wafer support pedestal is generally greatest in the center of the wafer and less towards the edges. Since the fluid temperature is generally uniform inside the wafer support pedestal, the wafer cools more rapidly in the center. This causes a temperature gradient across the wafer surface, becoming more severe with increased diameter wafers, e.g., 300 mm. This temperature gradient is one of the primary causes of feature variation in semiconductor wafer processing.
Another method of controlling wafer temperature that provides rapid dynamic control of the pedestal temperature uses thermoelectric devices embedded in the pedestal surface that supports the wafer (i.e., the support surface). These devices are oriented in a planar array below the support surface of the pedestal. However, within such an array, temperature gradients form between the individual devices, i.e., each device effectively transfers heat at its location while a lesser amount of heat is transferred at the locations immediately adjacent to and between the devices. Such gradients between a plurality of devices cause substantial temperature variation across the wafer, i.e., hot and cold locations are formed. Consequently, process variations may occur across the wafer in response to the temperature variations.
Therefore, there is a need in the art for an apparatus which can maintain a wafer at a predetermined temperature and provide a uniform temperature across the wafer support surface of a wafer support pedestal.